AsyncTask源码分析

上一篇我们分析了Handler的源码，这一篇我们来看一下AsyncTask，AsyncTask本身也是通过Handler，Message的机制来实现的。具体的实现方式，我们来看源码。

public AsyncTask() {
    mWorker = new WorkerRunnable() {
        public Result call() throws Exception {
            mTaskInvoked.set(true);
            Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
            //noinspection unchecked
            Result result = doInBackground(mParams);
            Binder.flushPendingCommands();
            return postResult(result);
        }
    };

    mFuture = new FutureTask(mWorker) {
        @Override
        protected void done() {
            try {
                postResultIfNotInvoked(get());
            } catch (InterruptedException e) {
                android.util.Log.w(LOG_TAG, e);
            } catch (ExecutionException e) {
                throw new RuntimeException("An error occurred while executing doInBackground()",
                        e.getCause());
            } catch (CancellationException e) {
                postResultIfNotInvoked(null);
            }
        }
    };
}

在构造函数中，我们先看到了构造了mWorker，再通过mWorker再构造了一个mFuture，就没有其他的了，这两个对象是用来干什么的先放在这里，稍后会看到。在我们想要执行AsyncTask的时候就需要我们执行execute，我们再从这个方法看起：

public final AsyncTask execute(Params... params) {
    return executeOnExecutor(sDefaultExecutor, params);
}

直接委托给executeOnExecutor来处理

public final AsyncTask executeOnExecutor(Executor exec,
        Params... params) {
    if (mStatus != Status.PENDING) {
        switch (mStatus) {
            case RUNNING:
                throw new IllegalStateException("Cannot execute task:"
                        + " the task is already running.");
            case FINISHED:
                throw new IllegalStateException("Cannot execute task:"
                        + " the task has already been executed "
                        + "(a task can be executed only once)");
        }
    }

    mStatus = Status.RUNNING;

    onPreExecute();

    mWorker.mParams = params;
    exec.execute(mFuture);

    return this;
}

这里我们看到了onPreExecute()的执行，这里也证明了为什么AsyncTask必须在UI线程中创建。再需要我们找的就是和是调用了doInBackground，exec是传入的sDefaultExecutor，sDefaultExecutor是SERIAL_EXECUTOR，是

SerialExecutor()对象，exec.execute(mFuture)，会执行r.run();也就是mFuture的run方法；
Executor sDefaultExecutor = SERIAL_EXECUTOR;
Executor SERIAL_EXECUTOR = new SerialExecutor();

private static class SerialExecutor implements Executor {
    final ArrayDeque mTasks = new ArrayDeque();
    Runnable mActive;

    public synchronized void execute(final Runnable r) {
        mTasks.offer(new Runnable() {
            public void run() {
                try {
                    r.run();
                } finally {
                    scheduleNext();
                }
            }
        });
        if (mActive == null) {
            scheduleNext();
        }
    }

    protected synchronized void scheduleNext() {
        if ((mActive = mTasks.poll()) != null) {
            THREAD_POOL_EXECUTOR.execute(mActive);
        }
    }
}

这里是FutureTask的run方法

public void run() {
    if (state != NEW ||
        !U.compareAndSwapObject(this, RUNNER, null, Thread.currentThread()))
        return;
    try {
        Callable c = callable;
        if (c != null && state == NEW) {
            V result;
            boolean ran;
            try {
                result = c.call();
                ran = true;
            } catch (Throwable ex) {
                result = null;
                ran = false;
                setException(ex);
            }
            if (ran)
                set(result);
        }
    } finally {
        // runner must be non-null until state is settled to
        // prevent concurrent calls to run()
        runner = null;
        // state must be re-read after nulling runner to prevent
        // leaked interrupts
        int s = state;
        if (s >= INTERRUPTING)
            handlePossibleCancellationInterrupt(s);
    }
}

记得我们在构造mFutureTask的时候传入了WorkerRunnable的对象，就是mWorker。这里的callable就是mWorker，c.call调用的就是mWorker.call。

public FutureTask(Callable callable) {
    if (callable == null)
        throw new NullPointerException();
    this.callable = callable;
    this.state = NEW;       // ensure visibility of callable
}

我们再回到AsyncTask的构造函数里，mWorker.call里我们看到了doInBackground(mParams)，这里我们看到了指定的第三个泛型Result result，作为执行后的结果 postResult(result)

private Result postResult(Result result) {
    @SuppressWarnings("unchecked")
    Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,
            new AsyncTaskResult(this, result));
    message.sendToTarget();
    return result;
}

到这里就是我们比较熟悉的发送Message到Handler，拿到的Handler是

sHandler = new InternalHandler();

处理消息的过程自然是在InternalHandler的handleMessage里

public void handleMessage(Message msg) {
    AsyncTaskResult result = (AsyncTaskResult) msg.obj;
    switch (msg.what) {
        case MESSAGE_POST_RESULT:
            // There is only one result
            result.mTask.finish(result.mData[0]);
            break;
        case MESSAGE_POST_PROGRESS:
            result.mTask.onProgressUpdate(result.mData);
            break;
    }
}

我们可以想象到MESSAGE_POST_RESULT 对应的就是处理完成的Message，而MESSAGE_POST_PROGRESS就是处理过程中，标示进度的消息，而发送这条消息的地方就是在publishProgress

protected final void publishProgress(Progress... values) {
    if (!isCancelled()) {
        getHandler().obtainMessage(MESSAGE_POST_PROGRESS,
                new AsyncTaskResult(this, values)).sendToTarget();
    }
}

finish里一定就是onPostExecute，忘了还有onCancelled

private void finish(Result result) {
    if (isCancelled()) {
        onCancelled(result);
    } else {
        onPostExecute(result);
    }
    mStatus = Status.FINISHED;
}

整个过程我们就分析完了，AsyncTask的本质还是thread + handler message 实现的，我们也可以理解为什么创建AsyncTask为什么一定要在UI线程中。